Suspended lattice for electrical interconnects

ABSTRACT

A method of forming an electrical interconnect includes etching a lattice pattern into a contact pad on a circuit substrate as a first connection point, at least partially filling the lattice pattern with a conductive epoxy, contacting the conductive epoxy with a second connection point, and curing the epoxy.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. patent application Ser. No.13/352,215, filed Jan. 17, 2012, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

This disclosure relates to electrical interconnects, more particularlyto electrical interconnects using conductive epoxy.

BACKGROUND

Conductive epoxy provides many advantages in bonding different layers ofan electrical apparatus. For example, print or other liquid dispensingtechniques can dispense the epoxy, allowing for larger scale or quickerpreparation in bonding two structures together, while creatingelectrical connections. Many epoxies retain some elasticity aftercuring, offering advantages for flexible circuit substrates.

Flexible interconnects often take the form of metalized pads serving asa bond point for the conductive epoxy. These pads are flat metal areason each of the surfaces undergoing bonding. The amount of conductiveepoxy used often represents a failure point in creating a robustelectrical connection. If too little epoxy is used, the connection tendsto fail during thermal cycling. The epoxy typically separates from oneor the other surface to which it is attempting to bond. If too muchepoxy is used, the epoxy squeezes out beyond the intended conductivesurfaces and may cause electrical shorts to other conductive surfaces.

SUMMARY

An embodiment is a method of forming an electrical interconnect thatincludes etching a lattice pattern into a contact pad on a circuitsubstrate as a first connection point, attaching a coverlay to thecircuit substrate such that that connection point resides at leastpartially in the coverlay material, at least partially filling thelattice pattern with a conductive epoxy, contacting the conductive epoxywith a second connection point, and curing the epoxy.

Another embodiment is a method of forming an electrical interconnectthat includes etching a lattice pattern into a contact pad on a circuitsubstrate as a first connection point, at least partially filling thelattice pattern with a conductive epoxy, contacting the conductive epoxywith a second connection point, and curing the epoxy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side view of a prior art electrical interconnectconfiguration.

FIG. 2 shows a side view of a suspended lattice electrical interconnect.

FIG. 3 shows a cross-sectional view of a portion of the suspendedlattice electrical interconnect shown in FIG. 2.

FIG. 4 shows a top view of an example lattice pattern in the electricalconnection point of the electrical interconnect shown in FIG. 2.

FIGS. 5A-5G show additional example lattice patterns.

FIG. 6 describes a method of forming an electrical interconnect.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a prior art electrical interconnect 100 in a thermallystressed circuit that has a surface bond between two electricalconnection points 102, 104. The first electrical connection point 102 issecured to a circuit substrate 106. Anchor points 110 of the firstelectrical connection point 102 reside in a coverlay 112. An adhesivelayer adheres the coverlay 112 to the second electrical connection point104. The surface bond of this prior art electrical interconnect 100 is atwo-dimensional layer of conductive epoxy 114 sandwiched between thefirst electrical connection point 102 and the second electricalconnection point 104. This two-dimensional structure requires a criticalamount of conductive epoxy 114 to create a robust electrical connectionbetween the two electrical connection points 102, 104. Applying toolittle epoxy causes the electrical interconnect to fail during thermalcycling and sometimes separation occurs between the electricalconnection points as a result. If too much epoxy is applied, electricalshorts occur in nearby connections. Finding the balance between toolittle and too much epoxy is challenging, especially as the demand forincreased interconnect density rises.

FIG. 2 shows an electrical interconnect 200 that has a suspended latticebond between two electrical connection points 202, 204. In this example,the suspended lattice bond includes a lattice pattern that creates athree-dimensional bond between the two electrical connection points 202,204, rather than a two-dimensional bond as shown in FIG. 1 describingthe prior art. The three-dimensional suspended lattice-pattern bondallows for a greater volume of conductive epoxy to be applied betweenthe electrical connection points 202, 204, which increases therobustness of the connection during thermal cycling. Further, thethree-dimensional configuration decreases the precision necessary fordetermining the amount of conductive epoxy 206 that is required tostrike the critical balance between too little and too much conductiveepoxy 206 for the electrical interconnect and overall increases thethermal reliability of the electrical interconnect. Some commonconductive epoxies include carbon-tube impregnated epoxy and silverepoxy, although the suspended lattice bond interconnect increases thereliability of any electrically-conductive bonded material.

FIG. 2 shows a circuit substrate 208 having an electrical connectionpoint 202 on it. The circuit substrate 208 includes a gap 210 (cf. thesolid circuit substrate shown in the prior art example shown FIG. 1).The electrical connection point 202 has a lattice 212 of conductivematerial that is adjacent to or otherwise positioned within at least aportion of the gap 210. The lattice 212 is attached to the circuitsubstrate 208 by anchor points 214 in the example shown in FIG. 2. Inthis example, the electrical connection point 202, and thus the lattice212, is round and may have any suitable number of anchor points 214along its perimeter.

The conductive epoxy 206 encapsulates the lattice 212 and extendsthrough portions of the lattice 212 in some examples, as shown in thecross-sectional view of FIG. 3. This configuration forms athree-dimensional bond between the electrical connection points,although the second electrical connection point 204 is not shown in FIG.3. Referring back to FIG. 2, the height of the conductive epoxy 206 mayexceed the height of the electrical connection point 202. FIG. 2 showsthe conductive epoxy 206 extending along a majority of a top surface 216and an opposing, bottom surface 218 of the electrical connection point202, encapsulating the lattice 212 of conductive material, whichincreases the surface area of the connection between the conductiveepoxy 206 and the electrical connection point 202 from the prior artsurface bond configuration shown in FIG. 1.

FIG. 4 shows a top view of the electrical connection point 202 with acriss-cross patterned lattice 212 that defines multiplerectangular-shaped openings into which the conductive epoxy 206 isplaced. The electrical connection point 202 is oval-shaped in thisexample, although it can be any suitable shape in alternativeconfigurations. A coverlay material 220 extends along the perimeter ofthe electrical connection point 202 in this example.

FIGS. 5A-5G show alternative lattice 212 patterns for the electricalconnection point 202. The lattice 212 patterns in FIGS. 5A-5C includemultiple round openings 222 in various configurations. Morespecifically, FIGS. 5A-5C include multiple circular-shaped openings 222,spaced apart from one another and arranged in 3 columns in which thetwo, aligned outer columns 224 have four circular openings while themiddle column 226 has five circular openings that are offset from theopenings in the two outer columns 224. The radius of the circularopenings 222 is uniform within each example lattice pattern, althoughthese circular openings 222 vary in radius size in each of theillustrated examples shown I FIGS. 5A-5C. The lattice 212 patterns inFIGS. 5D-5G include two linear portions 228 that intersect each other atan approximately 90° angle and define four quadrant openings. The widthof each of the intersecting linear portions 228 is approximately equalwithin each of the specific example lattice patterns, although the widthvaries in each of the examples shown in FIGS. 5D-5G.

Referring again to FIG. 2, the example electrical interconnect alsoincludes a second electrical connection point 204 that is adjacent tothe electrical connection point 202 that is on the surface of thecircuit substrate 208. The second electrical connection point 204contacts the conductive epoxy 206 in any suitable manner, although FIG.2 shows that a top surface 230 of the second electrical connection point204 is in contact with a bottom surface 232 of the conductive epoxy 206.

A coverlay material 220 is in contact with the circuit substrate 208 andmay be positioned as a layer between the circuit substrate 208 and thesecond electrical connection point 204, as shown in FIG. 2. The anchorpoints 214 of the lattice 212 may reside in the coverlay material 220and may restrict the conductive epoxy 206 to the lattice 212 ofconductive material. An adhesive 234 may adhere the coverlay material220 to the second electrical connection point 204 in some examples, suchas the electrical interconnect 200 shown in FIG. 2.

The gap 210 in the circuit substrate 208 is approximately equal in widthto a gap 236 in the coverlay 220 and adhesive layer 234. The gaps 210,236 form a continuous column between the circuit substrate 208, thecoverlay 220, and the adhesive layer 234 with the electrical connectionpoint 202 extending therebetween. The conductive epoxy 206 extends fromthe top surface of the second electrical connection point 204 to aheight approximately extending to the top surface of the circuitsubstrate 208. Because the conductive epoxy 206 extends through thelattice 212, the electrical interconnect 200 can withstand greaterthermal cycling without damage or separation occurring between theelectrical connection points 202, 204 and the conductive epoxy 206.

The electrical interconnects described above can be made in manydifferent manners. FIG. 6 shows the steps of one example method offorming an electrical interconnect. A lattice pattern is etched into acontact pad on a circuit substrate 600. The lattice pattern is thenfilled with a conductive epoxy 602 and a second connection point is thencontacted with the conductive epoxy to interconnect the contact pad andthe second connection point 604. The epoxy is then cured 606 in anysuitable fashion. As described above, the circuit substrate may includea gap and the lattice pattern that is etched into the contact pad may bepositioned adjacent the gap in the circuit substrate. The conductiveepoxy encapsulates the lattice pattern, as described above withreference to FIG. 2. The lattice pattern includes any suitable pattern,such as one or more round or linear openings. A coverlay material may beapplied to the circuit substrate such that a portion of the contact padresides in the coverlay material.

It will be appreciated that several of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. That variouspresently unforeseen or unanticipated alternatives, modifications,variations, or improvements therein may be subsequently made by thoseskilled in the art which also are intended to be encompassed by thefollowing claims.

What is claimed is:
 1. A method of forming an electrical interconnect,comprising: etching a lattice pattern into a contact pad on a circuitsubstrate as a first connection point; at least partially filling thelattice pattern with a conductive epoxy; contacting the conductive epoxywith a second connection point; and curing the epoxy.
 2. The method ofclaim 1, wherein the lattice pattern is etched into the contact padadjacent a gap in the circuit substrate.
 3. The method of claim 1,wherein the conductive epoxy encapsulates the lattice pattern.
 4. Themethod of claim 1, wherein the lattice includes at least one roundopening.
 5. The method of claim 1, wherein the lattice includes twointersecting linear portion defining four quadrant openings.
 6. Themethod of claim 1, further comprising applying a coverlay material tothe circuit substrate such that a portion of the contact pad resides inthe coverlay material.
 7. The method of claim 6, further comprisingforming anchor points between the lattice and the coverlay material. 8.The method of claim 6, wherein at least partially filling the latticepattern with the conductive epoxy comprising filling the lattice patternwith the epoxy such that a height of the epoxy exceeds a height of thefirst connection point.
 9. The method of claim 1, wherein the latticepattern comprises an array of circular openings.
 10. The method of claim9, wherein each of the circular openings have a same radius.
 11. Themethod of claim 1, wherein the conductive epoxy comprises one of silverepoxy or carbon-tube impregnated epoxy.
 12. A method of forming anelectrical interconnect, comprising: etching a lattice pattern into acontact pad on a circuit substrate as a first connection point;attaching a coverlay to the circuit substrate such that that connectionpoint resides at least partially in the coverlay material; at leastpartially filling the lattice pattern with a conductive epoxy;contacting the conductive epoxy with a second connection point; andcuring the epoxy.
 13. The method of claim 12, wherein the latticepattern is etched into the contact pad adjacent a gap in the circuitsubstrate.
 14. The method of claim 12, wherein the conductive epoxyencapsulates the lattice pattern.
 15. The method of claim 12, whereinthe lattice includes at least one round opening.
 16. The method of claim12, wherein the lattice includes two intersecting linear portiondefining four quadrant openings.
 17. The method of claim 12, furthercomprising forming anchor points between the lattice and the coverlaymaterial.
 18. The method of claim 12, wherein at least partially fillingthe lattice pattern with the conductive epoxy comprising filling thelattice pattern with the epoxy such that a height of the epoxy exceeds aheight of the first connection point.
 19. The method of claim 12,wherein the conductive epoxy comprises one of silver epoxy orcarbon-tube impregnated epoxy.